Generally, plastics excel in transparency, light weight, processability and impact strength but show poor abrasion resistance and solvent resistance. Hence surfaces of plastic articles are easily injured and damaged by organic solvents. Furthermore, they have a defect of insufficient weatherability and are apt to be discolored and turn yellow in color. With the view to improve these shortcomings, various surface-modifying coatings and modifying methods have been proposed.
Japanese laid-open (kokai) Patent Publication No. 196237/1983 and 78240/1984 propose coated polycarbonate molded articles with improved abrasion resistance, which are prepared by applying onto a polycarbonate surface an undercoater containing a thermoplastic or thermosetting acrylic polymer, curing the same, and thereafter coating said surface with a composition containing a partially hydrolyzed condensate of alkyltrialkoxysilane or colloidal silica-containing organopolysiloxane, and curing the same to let it serve as the top coating layer.
However, the above polycarbonate molded articles are still insufficient in resistance to abrasion and solvents compared to inorganic materials; they show insufficient water-intercepting properties, viz., prevention of water permeation which is one of the degradation factors in such articles in weatherability tests such as outdoor exposure tests.
On the other hand, Japanese Patent Publication No. 8098/1986 and Japanese Laid-open Patent Publication No. 32133/1990 propose coating compositions and articles treated therewith, in which a coating containing a polyfunctional acrylic compound is applied onto plastic surfaces and then cured with irradiation of actinic rays.
Japanese Laid-open (Kokai) Patent Publication Nos. 1756/1983, 49036/1985, 11665/1990, 163134/1990 and 173163/1990 all disclose coating compositions containing a polyfunctional acrylic compound and silica sol, or a polyfunctional acrylic compound, silica sol and silylacrylate; or plastic coated articles on the surfaces of which the coating compositions are cured with irradiation of actinic rays.
The coated articles, prepared by applying a coating containing a polyfunctional acrylic compound on a plastic substrate and then curing the same with irradiation of actinic rays, do exhibit improved resistance properties to abrasion and solvents, which, however, are yet insufficient compared to inorganic materials. Furthermore, water-intercepting ability, viz., prevention of water permeation which is one of the degradation factors of such articles in weatherability tests such as outdoor exposure tests, is insufficient.
Japanese Laid-open International Patent Publication Nos. 500894/1980 and 500980/1880 disclose polycarbonate articles which are coated with an undercoating layer, composed of a photoreactive product of polyfunctional acrylic monomer, and a thin silicon dioxide-containing layer being vacuum evaporated on the undercoating layer. For practicing the vacuum evaporation method proposed in those International Applications, however, a vacuum-type apparatus is required for forming the silicon dioxide coating, and furthermore, it is difficult to form a uniform coating on the surface of a plastic molded article of large size or with an uneven surface using such appartus.
For forming silicon dioxide coating film, besides the above-described vacuum evaporation method or sputtering method, Japanese Laid-open (Kokai) Patent Publication No. 258480/1985 proposes a process comprising immersing a substrate material in an aqueous hydrosilicofluoric acid supersaturated with silicon dioxide to form a silicon dioxide coating on the substrate surface. (This method is hereinafter referred to as the "precipitation method".)
Japanese Laid-open (Kokai) Patent Publication No. 12734/1986 discloses a process for forming silicon dioxide film on plastic molded articles by the precipitation method, which comprises coating a plastic molded article with a silicon compound such as an organosilicon compound, hydrolyzate thereof and colloidal silica, curing the same to form a primary coating, and thereafter forming a silicon dioxide coating on said primary coating by the precipitation method.
According to this process, plastic molded articles coated with abrasion-resistant, solvent-resistant and water-impermeable silicon dioxide film can be obtained, but the use of such a process still encouters the problems to be solved, such as insufficient reproducibility in transparency and adhesion of the silicon dioxide coating so formed. That is, it is understood generally that, in cases of coating a silicon dioxide film on a surface of a plastic molded article directly by the precipitation method, the resultant silicon dioxide film shows weak adhesion and is not uniform, because aqueous hydrosilicofluoric acid solution has low reactivity with plastics and poor wetability. On the other hand, if the plastic molded article is coated in advance with an organosilicon compound and cured, the silanol groups which are present on the surface provide anchoring sites with the silicon component in the aqueous hydrosilicofluoric acid solution, and hence, it is possible to improve adhesion and unevenness of the silicon dioxide film obtained by the precipitation method. However, when a silicon dioxide coating is formed on a film prepared by coating and curing a partially hydrolyzed product of silicon compounds as above, by the precipitation method, the reproducibility of uniformity and adhesion of the silicon dioxide coating is insufficient.
An object of the present invention is to provide novel coated plastic molded articles.
A further object of the present invention is to provide coated plastic molded articles with improved abrasion resistance, weatherability, chemical resistance and imperviousness to water.
Still another object of the present invention is to provide a process for manufacturing the coated plastic molded articles of the present invention with excellent reproducibility and industrial advantage.
Other objects and advantages of the present invention will become apparent from the following descriptions.